One known arrangement of the cooling system of this type is to perform cooling by circulating the coolant through the cylinder head and cylinder block by means of a water pump driven by the internal combustion engine, as disclosed in the following non-patent document 1, for example. In this cooling system, the coolant after cooling the cylinder head and cylinder block is returned to the water pump through a radiator and a heater, etc. The returned coolant is supplied again to the cylinder head and cylinder block through the water pump.
Non-patent document 1: Service Manual HONDA ACCORD Section: Chassis Maintenance (2002, 10) Page 5-147, 5-156
In such a cooling system, however, when the cylinder head and cylinder block are cooled, part of the coolant is vaporized and the produced bubbles are transferred to the water pump. As a result, air drawing occurs in the water pump and thereby the cooling system is liable to lower the cooling performance.